chore(shell): powerlevel10k prompt, host-aware bootstrap, drop thefuck

- zsh prompt: replace oh-my-posh with powerlevel10k via zinit, with the
  instant-prompt preamble near the top of .zshrc for fast startup
- remove thefuck/thefk entirely: .zshrc aliases, install-thefk,
  home_config/thefuck, and its .stow-global-ignore entry
- link-dotfiles: one-shot host-aware bootstrap — detect macOS vs Linux,
  install GNU stow if missing (brew / apt/dnf/pacman), then stow both trees
- retire oh-my-posh: delete the bash + pwsh installers and
  home_config/ohmyposh (starship covers PowerShell separately)
- README: document the new bootstrap and prompt setup

link-dotfiles.ps1

@@ -6,7 +6,7 @@
 .DESCRIPTION
     stow can't run natively on Windows, so this mirrors what it does:
       home_root/*    ->  ~\<name>           (e.g. ~\.zshrc)
-      home_config/*  ->  ~\.config\<name>   (e.g. ~\.config\ohmyposh)
+      home_config/*  ->  ~\.config\<name>   (e.g. ~\.config\nvim)
 
     Directories are linked with junctions and files with symlinks/hardlinks,
     none of which need administrator rights (junctions and hardlinks never do;